Magnetic recording media are normally produced by laminating a magnetic laminate having a magnetic recording layer, protective layer and the like on a substrate, which is a non-magnetic support, by going through steps such as coating, transfer, lamination or the like. With the exception of the case of a coating step, a magnetic recording medium formed in this manner tends to be partially formed on a non-magnetic support. In such cases, in order to eliminate a level difference that forms between the portion where the laminate is formed and the portion where the laminate is not formed, hot pressing is carried out on the magnetic laminate after having formed the magnetic laminate on a non-magnetic support serving as the substrate to embed the magnetic laminate into the substrate and form the same smooth plane between the surface of the uppermost surface layer of the magnetic laminate, which is a protective layer, and the surface of substrate where the magnetic laminate is not formed.
Examples of methods used to form a magnetic laminate on a non-magnetic support serving as a substrate prior to hot pressing include a method in which the magnetic laminate is formed in a transfer step using mainly a transfer laminate, and a method in which the magnetic laminate is formed in a lamination step using a sticking label laminate, and protective layers directly exposed to hot pressing by a mirrored metal pressing plate are required to have various properties.
Examples of important functions required of the protective layers of thermal transfer laminates and sticking label laminates include seizure resistance, which prevents seizure on the pressing plate when subjected to hot pressing, and scratch resistance, which prevents scratches that can occur in the protective layer of a magnetic layer after it has become the uppermost layer of a magnetic recording medium. In addition, in the case of the protective layer of a thermal transfer laminate, the protective layer is required to have good releasing performance that allows it to peel easily from a transfer support during heat transfer to the substrate for the magnetic recording medium. In order to satisfy these functions, a binder resin is used in the protective layer that easily separates from the transfer support and has interlayer adhesion with the magnetic recording layer to ensure good releasing performance, while an additive such as wax is used to improve scratch resistance. Known examples of protective layer binder resins include acrylic-based resins, butyral-based resins and cellulose-based resins (see, for example, Patent Document 1), while known examples of additives for improving scratch resistance include fatty acids and salts thereof, various types of wax (see, for example, Patent Document 2) and fluororesin particles (see, for example, Patent Document 3).
In an actual production process of a magnetic recording medium, for example, after heat-transferring a laminate containing a magnetic recording layer on a transfer support onto a substrate, which is a non-magnetic support, the transfer support is peeled off, and hot pressing using a metal pressing plate polished to a mirrored finish is carried out on a laminate containing a magnetic recording layer wherein a protective layer has been transferred as the uppermost layer. Alternatively, a sticking label laminate is arranged on a substrate while providing an adhesive layer there between as necessary, and hot pressing is carried out that presses the substrate onto a protective layer using a metal pressing plate polished to a mirrored finish. In particular, numerous magnetic recording media referred to as magnetic cards, in which a magnetic stripe is arranged on a portion of a paper or plastic substrate by the above-mentioned processing methods, are produced and widely used throughout the world.
Polyethylene wax, which has conventionally been preferably used as a wax for improving scratch resistance, functions as a lubricant on the protective layer of a magnetic recording medium such as a magnetic card due to its own inherent superior scratch resistance and the lubricity of a low melting point wax component that is melted by heat during hot pressing, and imparts preferable durability for practical use as a magnetic recording medium by reducing wear of the protective layer caused by a magnetic head used for magnetic recording and reading.
However, when carrying out hot pressing using a metal pressing plate polished to a mirrored finish in a production process of a magnetic recording medium, a portion of the polyethylene wax component added to the conventional protective layer ends up migrating to the surface of the metal pressing plate, thereby resulting in the problem of soiling the pressing plate.
In addition, in the case of adding polytetrafluoroethylene (to also be abbreviated as PTFE) particles, which are a type of fluororesin particles conventionally known to be an additive for improving scratch resistance, as lubricant, due to the high melting point of the PTFE particles themselves, they do not migrate to the metal pressing plate having a mirrored finish surface during hot pressing, thereby allowing the obtaining of satisfactory lubricity-imparting effects. However, due to inferior adhesion with binder resin, PTFE particles end up degranulating from the binder resin during magnetic card production and during routine handling such as that during magnetic reading and writing, thereby making it difficult to obtain preferable durability for practical use as a magnetic recording medium.
Although it is effective to reduce the amount of polyethylene wax added or use a polyethylene wax having a low degree of migration in order to reduce migration of the polyethylene wax component to the metal pressing plate as described above, in any of such cases, surface lubricity of the protective layer decreased and it became difficult to obtain preferable durability for practical use as a magnetic recording medium.
In this manner, when hot pressing is carried out using a metal pressing plate polished to a mirrored finish in a production process of a magnetic recording medium as described above, if migration of a wax component used as a lubricant into the metal pressing plate is attempted to be inhibited, it is difficult to obtain durability for use as a magnetic recording medium. On the other hand, when scratch resistance of the magnetic recording medium to a magnetic head is attempted to be improved, there was the problem of the additive used for that purpose, and mainly the wax component used as a lubricant, ending up migrating to the surface of the metal pressing plate of the press. In the case of continuously carrying out hot pressing in particular, the migrated component ended up accumulating, causing the accumulated component to transfer to the magnetic recording medium, or leave a mark or cause other problems such as soiling or deforming a pattern formed on the magnetic recording medium. Consequently, it was necessary to periodically clean the surface of the metal pressing plate thereby causing a decrease in production efficiency.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H7-65356 (page 3)    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-236637 (page 4)    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2001-351074 (page 3)